A process for preparing an amine compound by means of reaction of ammonia or a primary or secondary amine compound with a carbonyl compound has been known as a reductive amination reaction, which is one of the standard processes for preparing amine compounds. As preparation processes of amine compounds by means of reductive amination reaction, the following processes have been conventionally known: 1) a method by a hydrogenation reaction using solid catalysts such as Raney Ni, Raney Co, Pt/activated carbon, and Pd/activated carbon as a heterogeneous catalyst, 2) a method wherein a boron reactant such as NaBH3CN or NaBH (OAc)3 is used as a hydride reducing agent, and 3) a method wherein a metal complex catalyst is used as a homogeneous catalyst. The method of 1) is described in, for example, J. Am. Chem. Soc. 1941, 63, 749 and J. Org. Chem. 1962, 27, 2205.
Regarding the method of 2), the following methods have been known: a) a method using NaBH3CN described in J. Am. Chem. Soc. 1971, 93, 2897, b) a method using NaBH(OAc)3 described in J. Org. Chem. 1996, 61, 3849, c) a method using pyridine borane described in J. Org. Chem. 1995, 60, 5995, d) a method using 2-picoline borane descried in JP2004256511 (2004), and e) a method using 5-ethyl-2-methylpyridine borane described in Tetrahedron Letters 2008, 49, 5152-5155.
Regarding the method of 3), the following methods have been known: in Patent Literature 1, a method for preparing a primary amine by reaction of a carbonyl compound, ammonia and hydrogen under the presence of a hydrogenation catalyst; in Non-patent Literature 1, a method by means of hydrogenation reaction using a rhodium complex having a phosphine ligand; in Non-patent Literature 2, a method by means of hydrogenation reaction using a combination of a [Rh(cod)Cl]2 complex and a TPPTS ligand; in Non-patent Literature 3, a method using [Cp*RhCl2]2 as a complex catalyst wherein ammonium formate is used as an amine source and a hydrogen source; in Patent Literature 2, a method by means of reductive hydride-transfer animation of a carbonyl compound with an amine compound under the presence of a hydrogen donator as a reducing agent and a transition-metal complex catalyst comprising at least one metal selected from the group consisting of Ru, Rh, Ir.
However, the preparation processes by means of hydrogenation reaction using solid catalysts have problems in terms of safety and operability of reaction, because they require a pressure-resistant reactor due to the use of hydrogen gas as a hydrogen source, and also the processes cannot be applied to substrates having carbon-carbon multiple bonds and functional groups such as cyano group and nitro group that are apt to be hydrogenated. Since boron reactants do not require a pressure-resistant reactor, they will make a method with superior operability; however, because it is not a catalyst reaction, the method is inferior from the viewpoints of economical and environmental aspects. In addition, the following problems exist for each reactant.
The method using NaBH3CN has a difficulty in industrial utilization due to its toxicity. The method using NaBH(OAc)3 has a limitation in solvents used due to its solubility, and an excessive amount of NaBH (OAc)3 must be used because there is only one hydride source in its molecule. Regarding the method using pyridine borane, the storage stability of the reagent itself is poor, and has a problem that it decomposes at 54° C. or more. The method using 2-picoline borane has a problem of handling because its melting point is 44-45° C., although the stability of the reactant is higher than that of pyridine borane. 5-ethyl-2-methylpyridine borane has a problem that the reactant is difficult to be removed from a reaction liquid, similar to the above two pyridine-borane reactants.
As an example wherein a homogeneous catalyst is used, a method described in JP No. 4059978 requires an pressure-resistant reactor because it uses hydrogen as a hydrogen source, and reaction is performed under high-temperature (150° C.) and high-pressure (50 atm or more) conditions; therefore the method has problems in industrial application in terms of safety and reaction operability. A method described in Chem. Comm., 2000, 1867-1868 is a reaction under high-pressure condition (50 atm), and it generates alcohols as a by-product and has deteriorated selectivity of amines. Thus, the method cannot be an efficient preparation process of amine compounds. A method described in Org. Lett. 2002, 4, 2055-2058, that is, a hydrogenation reaction of combination of a [Rh(cod)Cl]2 complex and a TPPTS ligand, is performed under high temperature and high pressure conditions; therefore, its industrial application has difficulties in terms of safety and reaction operability.
As shown in J. Org. Chem. 2002, 67, 8685-8687, a method wherein [Cp*RhCl2]2 complex as an organometallic complex is used as a catalyst is superior in terms of reaction operability and safety, because it uses solid ammonium formate as a hydrogen source and amine source. However, because catalyst activity is low, its industrial application may be problematic. JP A, 2004-537588 describes a method that has difficulties in industrial application due to its low reaction efficiency, such as a [substrate/catalyst] ratio of approximately 50-100, and the generation of by-product alcohols and a possibility of incomplete reaction.
Under these circumstances, a novel preparation method of amine compounds by means of reductive amination reaction, using a catalyst for general use having high activity and superior functional-group selectivity, has been conventionally desired.